Apps have been all the rage in the social impact world recently. We've reported on apps that trigger donations when you wash clothing or run a mile, and others that pause your text conversation while driving or help you make responsible purchasing decisions. The newest effort harnesses app technology to pit celebrities against each other in a competition to raise money for a cause, while reinforcing healthy lifestyles.

This week, Fitbit partnered with the American Heart Association (AHA) for a new effort to raise money and promote heart health. The Fitbit Celebrity Challenge asks two celebrities to track their steps for one week using Fitbit trackers. The person with the most steps is the Challenge winner, triggering a $10,000 donation to AHA. To kick things off, Victoria's Secret and Sports Illustrated models Erin Heatherton and Hilary Rhoda are facing off to amass as many steps as possible in a week. Next up, San Francisco Giants outfielder Hunter Pence and Miami Marlins infielder Dee Gordon will compete step for step. To get consumers involved and to encourage a little healthy competition, Fitbit's website allows individuals to boost their favorite celebrity's count by 10 steps for each dollar donated to AHA. Fitbit also encourages users to create their own challenges with friends and family to increase physical activity and prevent cardiovascular disease and stroke through walking. Earlier this month the wearable technology company also launched FitForFood, a challenge encouraging Fitbit users to join comedian Joel McHale in an effort to burn a collective 1 billion calories, triggering a donation of 1.5 million meals to Feeding America.

Apps present a major opportunity for brands to increase engagement in programs or encourage specific behaviors – and consumers are primed to participate. More than half of Americans, and an overwhelming three-quarters of Millennials, say they'd use apps to be rewarded for behavior change or donate to a cause. Fitbit's Celebrity Challenge offers a refreshing twist on this new trend, using celebrity popularity and a little healthy competition to increase donations and boost health.